The proposed invention relates to a synergistic process involving simultaneous production of organic acids and leaching of nickel and other base metals from ores and waste products. In particular, the present invention relates to a combination of citric acid production process with a metals leaching and recovery process to yield a synergistic and low cost opportunity with significant economic potential.
With fewer high-grade nickel resources being discovered, the development of alternative technologies has become the primary goal of the nickel production industry. The natural resources that are mined for nickel production include ores where the nickel originates from a magmatic source and, therefore, is tied up with sulfur in (mixed) sulfide ores. Sulfide ores are generally rather deeply located and often underground mining is required to obtain them. These resources range in grade from 0.5% to about 2% Ni. Non-magmatic nickel resources are the so-called nickel laterites. These consist of weathered ultra basic rocks in which the low level nickel concentration of the parent rock is concentrated to economic values through geological weathering processes, resulting in heterogeneous mixtures of hydrated iron oxides (goethite) and hydrous magnesium silicates, which both contain low concentrations of nickel and cobalt. Since laterites are generally surface or near-surface deposits, low cost open cut mining techniques are used to mine the ore.
Laterite ore bodies generally consist of two main zones: a high-iron limonitic zone with about 1.3% Ni and 0.1% Co and a lower silicate ore zone, called saprolite, where continued weathering takes place with nickel values as high as 3%, but with little cobalt. In the transition from the limonitic zone to the saprolite zone, a third, relatively minor ore occurs, consisting of various magnesium silicate clays. All three of these ore types contain considerable amounts of absorbed water, typically 35 to 50 percent by weight, which has an important effect on the processing method.
In the upper or limonite zone, the nickel is present in the iron oxide phase. A separate nickel phase has never been identified. It is not completely clear whether the nickel is chemically bound in the goethite (FeOOH) matrix or adsorbed onto the surface. In the lower or saprolite zone, the nickel tends to be concentrated in a weathering product, mainly serpentine, or (Mg,Ni)3(Si2O5)(OH)4, and other magnesium silicate minerals, with nickel partially replacing magnesium.
Typical chemical compositions of the three types of ores are given in Table 1. Since the transition and the saprolite zones have a similar weathering profile involving the conversion of nickeliferous silicates to iron oxides, the two profiles are usually combined.
Geologists are not certain how nickel and magnesium are transported during the weathering (xe2x80x9clateritizationxe2x80x9d) process, but it is generally believed that a low pH is required. Carbonic acid has been thought of as the chemical that could supply the required hydrogen ions as well as provide for the low pH. However, carbonic acid by itself does not generate pH values lower than around 4.5. This may not be adequate for the chemical weathering process. It is now being postulated that both the cation ion (H+) as well as the anion of the acid must play a role in mineral dissolution and element transport and is facilitated by metal complexation rather than as free positively charged ions (Ni2+ and Mg2+). Weathering by itself also happens in an environment where vegetation and its breakdown products will play a complementary role in the mineral weathering process. Organic material is readily converted by bacteria and other organisms to organic acids such as humic and other carboxylic acids. Some plants can actually act as metal accumulators, especially when they metabolize with water that contains organic anion-complexed metal ions.
Since, relative to the underlying bedrock, nickel is enriched in the limonite and serpentine phases during lateritization, the basis of this invention is the reverse of the chemical reaction that resulted in the formation of an ore with an economic nickel content. In other words, the process according to the present invention seeks to reverse what Mother Nature has previously done: de-adsorb the nickel from its ore. In essence a possible reaction mechanism can be written as follows:             serpetine      +              2        ⁢                  xe2x80x83                ⁢                  Me                      x            +                                =                  magnesium        ⁢                  xe2x80x83                ⁢        silicate            +              x        ⁢                  xe2x80x83                ⁢                  Ni                      2            +                          ⁢                  xe2x80x83                ⁢        or                                                      (                          Mg              ,              Ni                        )                    3                ⁢                  (                                    Si              2                        ⁢                          O              5                                )                ⁢                              (            OH            )                    4                    +                        3          y                ⁢                  xe2x80x83                ⁢                  Me          y                      2            +                          ⁢                  L          2                      y            -                                =                                        (                          Mg              ,              Me                        )                    3                ⁢                  (                                    Si              2                        ⁢                          O              5                                )                ⁢                              (            OH            )                    4                    +                        3          y                ⁢                  xe2x80x83                ⁢                  Ni          y                ⁢                  L          2                      y            -                              
where xe2x80x9cLxe2x80x9d represents an organic anion, such as citrate.
The technical viability of using organic acids to dissolve nickel from laterite ore is shown in FIG. 5 where a comparison between atmospheric sulfuric and citric acid leaching is made.
To address the issues of the low Ni grades of many laterites, cost-effective processing, and environmental concerns, the use of heterotrophic microorganisms for microbial leaching of nickel from lateritic ores has been investigated. The work has generally focused on leaching with fungi related to the Aspergillus and Penicillium genera. These types of fungi produce hydroxy-carboxylic acids such as citric acid and oxalic acid as metabolites, which have shown to be able to dissolve nickel, cobalt, and iron from a variety of laterite ores. While work has been done to determine the effectiveness of some heterotrophic bacteria for nickel leaching, the bacteria were found to be much more easily affected by nickel poisoning than the fungi.
Heterotrophic microorganisms are different from autotrophic microorganisms in that they require organic carbon for growth rather than carbon dioxide. Also, they do not directly degrade the mineral, but rather the degradation is achieved via the production of acid metabolites, such as citric acid and oxalic acid. It is thought that the metal extraction by microbially produced acids occurs through two mechanisms, which can take place separately or simultaneously. The first mechanism involves the displacement of metal cations from the ore matrix by hydrogen ions, resulting in dissolution of the metal ions, while the second is thought to be based on the ability of the organic acids that are produced to form soluble complexes with the metal ions by chelation.
By using an organic acid lixiviant to solubilize base metals rather than conventional mineral acids, environmental issues related to contamination of heaps, tailings and soils will be minimized. Neutralization of any residual acid may not be required as microbes can readily oxidize the remaining organic acid.
Therefore, the objects of the present invention are to provide a bioleaching process for nickel from lateritic ores; and to provide a process of acid productionxe2x80x94in-situ or ex-situxe2x80x94and method for recovering metals.
The present invention provides a process for the co-production of citric acid and solublized metals, particularly nickel and cobalt. According to the process of the present invention, a microorganism, capable of producing an organic acid selected from the group consisting of oxalic acid, humic acid, citric acid, and mixtures thereof, is mixed with an ore containing a base metal, preferably at least nickel and optionally cobalt. The mixing occurs under conditions to leach the nickel from the ore to produce a solution rich in nickel. The solution preferably contains a nickel salt and a magnesium salt. The nickel salt can be separated from the magnesium salt and thereafter, the nickel can be recovered in any suitable known manner. The organic metal salt produced as a result of the mixing can be either sold or used and converted into an organic acid.
In one embodiment of the present invention, a microorganism capable of producing an organic acid selected from the group consisting of oxalic acid, humic acid, citric acid, and mixtures thereof, is mixed with a nutrient and an ore. While mixing, the microorganism produces an organic acid, which when produced, leaches base metals from the ore. The leached metals are recovered in any suitable known manner. The organic anion can be recovered while the leach solution (i.e., the microorganisms and nutrients), which is now barren, can be recycled and mixed with fresh microorganism, nutrient, and ore.
In a preferred embodiment, the microorganism produces citric acid in an indirect fashion and then leaches nickel and other metals from the ore. The nickel is recovered in a known manner. Likewise, the citric acid, which leached the nickel, also leached magnesium oxide present in the ore to produce magnesium citrate, which can be recovered or further treated to produce citric acid crystals and magnesium oxide for commercial sale. As a result, the process of the present invention provides a complete, economical, and environmentally friendly process for obtaining nickel from ore.
Unless otherwise noted, all percentages are by weight.